Registration page with PHP & MySQL - php

Question

I have a registration page, which is tied to this process.php code below. When I run this code, it returns "Error". Did I make a mistake somewhere?
<?php
require_once ('newmeowconnection.php');
if (isset($_POST['form_input']) && $_POST['form_input'] == 'registration') {
registerUser();
}
function registerUser() {
$query = "INSERT INTO users (first_name, last_name, email, password, created_at, updated_at)
VALUES('{$_POST['first_name']}','{$_POST['last_name']}','{$_POST['email']}', '{$_POST['password']}', NOW(), NOW())";
$run = mysqli_query($query);
if ($run) {
$_SESSION['loggedin'] = TRUE;
$_SESSION['user'] = $_POST['email'];
header('Location: http://localhost/homepage.php');
} else {
echo 'Error';
}
}
?>

Answer 1

mysqli_query need run on connection object or pass connection to it:
$run = mysqli->query($connection, $query);
or
$run = $connection->query($query);

Answer 2

The problem is you are using single quotes-inside single-quotes. For instance '{$_POST['first_name']}' is read as {$_POST[ being one thing first_name as a SQL variable and ]} another string.
Try the following
...
$first_name = $_POST['first_name'];
$last_name = $_POST['last_name'];
$email = $_POST['email'];
$password = $_POST['password'];
$query = "INSERT INTO users (first_name, last_name, email, password, created_at, updated_at) VALUES('{$first_name}','{$last_name}','{$email}', '{$password}', NOW(), NOW())";
...